We have found an endogenous reducing substance in cultured cells which can replace ascorbate as a cofactor for prolyl hydroxylase. The presence of this substance appears to explain the phenomenon of ascorbate-independent proline hydroxylation which occurs in transformed BALB 3T3 cells and stationary phase L-929 cells. Transformed BALB 3T3 cells have also been examined with respect to their collagen phenotype and content of microtubules. Transformation with Kirsten sarcoma virus resulted in an alteration of the collagen phenotype. Contrary to previously published reports on studies using qualitative microscopic methods to examine microtubules, we found no decrease in the microtubule content of transformed cells and, in fact, in most transformed cells there was an increase.